Precision punch and die set



July 23, 1957 v. E. MATULAITIS PRECISION PUNCH AND DIE sm' 3Sheets-Sheet 1 Filed D96. 8, l954 y 23, 1957 v. E. MATULAITIS PRECISIONPUNCH AND DIE SET Filed Dec. 8. 1954 3 Sheets-Sheet 2 IN V EN TOR. Way?1, mfuzdara July 23, 1957 Filed Dec 8, 1954' V. E. MATULAlTlS PRECISIONPUNCH AND DIE SET 3 Sheets-Sheet 3 JZE 3 INVEN TOR.

PRECISION PUNCH AND DIE SET Victor E. Matulaitis, Franklin, Mich.,assignor to Elox Corporation of Michigan, Clawson, Mich.

Application December 8, 1954, Serial No. 473,071

1 Claim. (Cl. 219-69) This invention relates generally to an improvedmethod of making punch and die sets and particularly to a method andmeans for making punch and die sets by aremachining or, as it issometimes called, spark-machining, electrical-discharge-machining ormetal disintegration.

The conventional method of making a punching die set which usuallyincludes a punch, a die and a stripper plate, comprises machining eachelement separately from tool steel, hard alloy, tungsten carbide or thelike. the material is hard, like hardened alloy or carbide, themachining is diflicult and time consuming and consequently expensive. Ifthe die set is machined before the material is hardened, lapping andbarbering must be performed after heat treating and this is expensiveand time consuming.

It is the main object of my invention to provide a meth d and means formaking a die set quickly and cheaply,

V which die set will be capable of precision work.

Another object is to provide a method of making die sets havingrelatively greater perfection of fit between the punch and the diewhereby parts produced by the die set will be to closer tolerances thanthose produced by die sets made by conventional methods.

Still another object is to provide a method of making a complete punch,die and stripper plate set by arc-machining withthe use of only oneelectrode.

Other objects and advantages of my invention will become apparent fromthe following description and discussion. In the drawings accompanyingthe description, in which reference characters have been used todesignate like parts referred to herein,

Fig. 1 is a plan view of a typical part produced by a die set of thetype herein mentioned;

Fig. 2 is a pictorial representation of the electrode and workpiece asthey appear during the arc-machining operation for producing the die orthe stripper plate;

Fig. 3 is a sectional view of the finished die plate made as shown inFig. 2;

Fig. 4 is a pictorial representation of the electrode and workpieceasthey appear during the arc-machining operation for producing the punch;

Fig. 5 is a part-sectional view of the parts shown in Fig. 4;

Fig. 6 is a view, with the die plate and stripper plate shown insection, of a complete die set made in accordance with the invention;and

Figs. 7, 8 and 9 illustrate successive stages in the machining of apunch blank.

Referring to the figures, it may be seen that the part P, shown in Fig.1, is of simple form, such as may be employed in an automobile doorlatch assembly or the like. It is typical of the type of part producedin large quantl ies by a stamping operation and has been chosen forillustrative purposes, but it is desired to emphasize that the methodherein disclosed is not limited to making die sets for simple parts.Indeed, it is when dies are required for s atenr O Patented July 23,1957 manufacture of complex parts that my invention shows its greatestadvantage because of the tremendous saving in time and cost.

A typical part like that shown in Fig. 1 might have the followingdimensions: A% inch, B /z inch, C% inch, and D- /s inch. The machiningspecification on the blueprint for the part would probably carry thenotation, All fractional dimensions or .010, in accordance with normalmachine shop practice. This latitude in dimension or tolerance as it iscommonly called is appreciable, but the tolerance permitted in themanufacture of the die set itself is much closer as will be apparentfrom the following explanation.

In the conventional method of making the die set, the allowabletolerance may be totally or substantially expended in the making of thedie plate. The punch then must be made to exact or near exact size withpractically a tolerance of zero. Conversely, if the punch is made firstand the allowable tolerance used up there, the die plate must be made toprecision standards in order to obtain proper coordination of the punchand die and to secure proper dimensional tolerance on the finished part.In either case, a considerable amount of lapping and barbering isnecessary by a skilled die man when the punch and die are sheared in inorder to provide the clearances required for good stamping work.

It is standard practice to provide a radial clearance between the punchand die equal to 5% of the thickness of the stock to be punched. Forexample, if the part P is to be punched from stock .060 inch thick, aradial clearance or gap of .003 inch between the punch and die must beprovided, and in conventional practice this clearance is obtained bygrinding and lapping, the labor being done by skilled die makers. Inother words, it is necessary for the die maker to work to a tolerance of.003 inch despite the fact that the finished part carries a generoustolerance of .020 inch.

Obviously the manufacture of die sets by conventional means is timeconsuming and expensive because of the necessity for precision work onthe part of highly skilled operators as outlined above. In themanufacture of die sets in accordance with the teachings of myinvention, the need for such precision entirely disappears; consequentlythe time required for making a precision die set is very substantiallyreduced with corresponding reduction in cost.

In my method of making die sets, I utilize arc-machining apparatus ofthe type described and claimed in Me- Kechnie Patent 2,501,954, anddetailed description thereof will be omitted herein in the interest ofbrevity. in arc-machining, an electrode of cross-sectioned formsubstantially identical to that of the part being machined is used inconnection with a power supply to machine the workpiece by means ofelectrical discharges between the electrode and the workpiece in thepresence of suitable coolant fluid.

In Fig. 2, I have illustrated in pictorial form the machining setup usedfor making the die plate and the stripper plate in accordance with myinvention. The electrode 10 is, in most instances, the only tool thatneed be constructed. It is preferably made of free-machining brass tothe tolerance permitted by the finished part. The electrode has aportion 16 adapted to be held in a collet of an arc-cutting machine anda passage 13 for the passage of coolant through the electrode to the arczone.

The workpiece 12 which may be either the die plate or the stripperplate, is in this instance the stripper plate and is of hard,electrically conductive material such as alloy steel, tungsten carbideor the like. The holes 26 for accommodating fastening bolts may bemachined in the piece 12 by another arc-machining operation eitherbefore or after that being described. The hole 20 which is cut in theplate 12 by electrical-discharge between the electrode and the plate isthe hole through which the p nch Pa ses in e p n of h P P. In he casewhen the plate is the die, the hole 20 is the opening, through which thepart P is punched by the punchinactual manufacture of the part.

A power supply 14 having the required characteristics is connected tothe electrode and workpiece as shown in accordance with the teachings ofthe aforesaid M'cKechnie patent.

In the arc-machining process, the chips eroded from the workpiece When ahole such as 20 is being cut are flushed out of the hole around theoutside of the electrode by thecoolant which enters through theelectrode passage 18 under pump pressure. As these chips in theirsuspended state pass out through the extremely small clearance spacebetween the electrode-and the side of the hole in the workpiece,conductive bridges are formed which causes side-arcing between the sideof the electrode and the side of the hole. As the hole is machined, theupper portion thereof is naturally exposed to the sidearcing effect fora longer period of time than lower portions thereof; consequently,erosion of the hole around the electrode occurs progressively as theoperation proceeds. The hole then acquires a taper, shown in greatlyexaggerated form in Fig. 3. This taper is necessary in the finished dieas a relief to permit the punched part to drop out of the die, and inthe conventional method of making dies is the cause of painstaking andlaborious work as above mentioned. In my improved die-making method, thedesired taper is obtained automatically as the hole is cut. The punchmust necessarily have a corresponding taper, and this is obtainedautomatically as will be explained.

As above mentioned, the plate 12 of Figs. 2 and 3, is

actually the stripper, plate and it may be of the same hard material ofthe punch and die or if desired it may be of softer and cheaper stock.In either instance the method of making the die set in accordance withthe teachings of my invention is the same.

After the stripper plate is finished, the die plate 36 (Fig. 6) may bemachined, and this is done inidentical manner as described for thestripper plate.

In cases where the stripper plate is of relatively soft material, it issometimes desirable to make several stripper plates, and this can beaccomplished by the method described and all of the plates will besubstantially identical.

In instances where the punch is relatively long, several phantom platesmay be machined of soft stock and these used as electrodes for machiningthe punch and then discarded.

The punch is made by using the phantom plate or stripper plate(preferably the latter)v as the electrode, or as just mentioned, one orseveral phantom plates may be used as the electrode.

The plate 12, now the electrode, is secured by screws 33 to an electrodeholder 28 having a coolant passage 18 opening into an enlarged opening29 for accommodating the increased area necessary to accommodate thepunch.

The negative terminal of the power supply 14 is connected to the holder28, and the positive terminal is connected to the punch blank, which inthiscase is theworkpiece (see Fig. 4). The punch blank is usually of thesame hard material as the die plate.

As shown in Fig. 5, the plate 12 when used as an electrode for cuttingthe punch is mounted so that the surface 24 faces the punch blank 30.This assures that the surface 32 of the punch Will be identical in shapeand size to the electrode 10 and to the piece P. As'the electrode plate12 is fed toward the punch, the desired punch length will be machined onthe blank and the electrode will be eroded becauseof normal electrodewear.

Figs. 7, 8 and 9, show successive stages during machining of the punchblank, and reference to these figures will clarify the procedure. Inthese figures, the plate 12 may be the plate that is ultimately used asthe stripper plate or it may be a phantom plate which is discarded afterthe punch is machined.

The plate 12, as it is fed toward the punch blank 30 will erode thelatter as shown in Figs. 8 and 9. As the electrode approaches the blank,the area of the blank embraced by the opening 20 will, of course, not beeroded. The machining operation will therefore start cutting the blankat the plane D-D (Fig. 7) and will produce a surface D'-D (Figs. 8 and9) on the punch blank. As the punch blank feeds into the opening 20, theblank will be machined as shown in Figs. 8 and 9 and there will be anaccompanying change in the size of the opening 20 at the plane DD and atparallel planes such as C-C, etc., because of natural electrode wear.Because of the reverse taper of hole 20, there is very little, if any,side-arcing between the electrode and the workpiece as the machiningprogresses. It is clear then that the surface at plane D- will remainsubstantially the size originally cut, but as the blank is fed intotheelectrode hole, a taper will be machined on'the blank as theelectrode wears away at planes DD, C-C, etc; Fig. 8 illustrates anintermediate stage in the cutting of the punch, and Fig. 9 shows thecondition of the parts at the end of the operation as normallyconducted.

The wear of the electrode in its relation to the length of punchmachined can be calculated and thus, by proper correlation of thematerials and factors involved, the punch can be given the desiredamount of taper. If the material of the plate 12 is the same as that ofthe punch (for example, tungsten carbide), the electrode will wear awayat the plane of the bottom of opening 20 at about two-thirds of the rateof erosion of the workpiece. This electrode wear is not particularlyobjectionable if the plate 12 is to be used as the stripper for the dieset or if the plate is to be discarded. If the plate is to be used as astripper, it is, of course, necessary that the machining of the punch bediscontinued before the. blank 30 passes entirely through the plate12;,otherwise, there would be no stock left around the opening 20 toprovide stripper action. Fig. 9 shows the plate 12 and punch blank atthe conclusion of the machining operation of the punch.

In instances where an unusually long punch is required or no stripperplate is wanted, brass or other inexpensive material may be used for theplate 12. The brass, when used as an electrode, will wear much fasterthan harder mate-rial; consequently, several plate electrodes arenecessary to maintain the proper dimensions on the punch. If severalplates are used to make one punch, care mustv be exercised in making theplates to insure absolute accuracy in the size, shape and location ofthe hole 20 in each plate.

In instances where a relatively long punch is required, it has beenfound to be economical to first make anumber of brass phantom plates andone plate of. tool steelor hard carbide. The hard plate is then used forthe last 'rnachining operation on the punch blank and is then availablefor use as a stripper if required.

A single thick brass plate is not usually satisfactory for machining along punch because the brass wears away so fast that the taper of thepunch is increased beyond allowable tolerances. The use of severalthinner plates avoids this.

It will be observed that the parts of a matched'die set made as outlinedherein will have complementary taper but this taper will not bedetrimental in any way. When the parts-require sharpening, the punchanddie may be ground simultaneously and thus, proper clearance will bemaintained throughout their useful life.

If it is desired to produce a die set without taper but with properclearance between the parts, application of the teachings of mycopending application Serial No. 693,774

may be resorted to. Proper clearance between the die parts is readilyobtainable with arc-machining apparatus utilizing modern power circuits.With such circuits, such as those shown in my copending applicationSerial No. 459,704, almost any desired practical gap size can beobtained, say through the range of .0004 inch to .004 inch, and thisclearance range is suitable for the great majority of commerciallyproduced stampings.

If greater or less clearances are required, corresponding arc-gaplengths may be obtained through the control of electrode size, as willbe explained in my copending application Serial No. 666,076, filed June17, 1957.

I claim:

A method of forming a punch for a die set having a cross sectional shapewhich conforms at the free end to the shape and dimensions of an articleto be punched which comprises, providing a plate having a taperedopening therethrough having a cross sectional shape and dimensions at afirst face corresponding to the shape and dimensions of the article tobe punched with the sides of said opening diverging in proceeding to thesecond face of the plate, arranging said first face of the plate withsaid opening adjacent the end of an elongated metal punch blank,supplying electrical current to the punch blank and the plate whilethere is relative movement of the punch blank and the plate towards eachother, supplying a coolant through the opening in the plate for flushingeroded metal from the periphery of the punch blank and the boundary ofthe opening, continuing said relative movement of the punch blank andthe plate to are machine the punch blank with the side surfaces of thepunch blank diverging from each other in proceeding from a free end ofthe punch blank whereby the shape and cross sectional dimensions at thefree end of the punch blank correspond to the shape and dimensions ofthe opening in said plate at the first face.

References Cited in the file of this patent UNITED STATES PATENTS

